Method and apparatus for receiving FDMA signals

Abstract: A communication apparatus capable of measuring levels of carrier signals used in adjacent zones during telecommunication with a small circuit scale, and capable of simultaneously transmitting/receiving to FDMA-system signals through a plurality of communication channels. All of transmission signals received through the communication channels under use are wholly converted into an intermediate signal by a mixer. The intermediate signal is quadrature-detected (24) and then A/D-converted into digital I/Q-channel signals by A/D-converters (26, 27). Thereafter, the digital I/Q-channel signals are frequency-converted by frequency converting circuits(43, 44) in such a manner that center frequencies of these I/Q-channel signals become zero, two sets of I/Q-channel signals may be demodulated by two sets of demodulating circuits (28). Otherwise, one of these I/Q-channel signals is demodulated by a single demodulating circuit(28), and the other is level-detected by a level detecting circuit (45).

Description: The present invention generally relates to communication method/apparatus/system used in the FDMA (frequency division multiple access) transmission system. More specifically, the present invention is directed to a communication apparatus for receiving a carrier signal transmitted from another base station during telecommunication operation and/or for measuring a level of this carrier signal, to communication method/system for performing telecommunication operation between a plurality of base stations as well as mobile stations. Conventionally, the analog signal transmission systems have been widely utilized in the mobile telecommunication services such as portable telephones and automobile telephones. Very recently, the digital signal transmission systems are commercially available in which as the radio (wireless) accessing method, the TDMA (time division multiple access) system for time-multiplexing a plurality of users to transmit the information with a single carrier frequency is employed. In this TDMA system, the timing at which one user receives the transmission signal is present within a certain time range slot of 1 frame. The presently available mobile communication system is arranged by the cellular telephone system. Accordingly, a mobile station is required to have a judging function as to the cell to which the own mobile station belongs. To achieve this judging function, the reception levels of the signals transmitted from the plural base stations must be measured. When the same frequency as that of the signal under communication is used by another user belonging to another cell, the interference signal levels caused from this user would be increased. When the interference signal levels exceed a preselected SIR (signal-to-interference ratio), the present communication channel is switched into other communication channels with different frequencies. As a consequence, the mobile station is required to detect the levels of the carrier signals used in the peripheral monitoring zones which are designated by the base station during the telecommunication. In accordance with the TDMA transmission system, as illustrated in Fig. 1, the levels of the reception signals from the peripheral zones may be detected within the empty slot (namely, level detecting slot) provided other than the receiving slot and the transmitting slot. During this empty slot, the frequency of the frequency synthesizer is made coincident with the designation carrier for monitoring the level, the signal level detection is carried out within this empty slot, and a plurality of detected sampling values are averaged to obtain the level of the reception signals. As described above, since the empty slot is employed in the TDMA transmission system, a single receiver can detect the levels of the carrier signals having the different frequency from that of the carrier signal under communication. However, in the SCPC/FDMA (single channel per carrier/frequency division multiple access) system where a single channel with 1 carrier is used by 1 user, since the single channel is always used, two series of signal receiving units are required, as shown in Fig. 2, in order to detect the levels of the different carrier signals. That is, the signal received from the input terminal 11 is subdivided by the hybrid 12 into two signal portions which will then be supplied to the bandpass filters 13 and 14. The outputs from the bandpass filter 13 and 14 are frequency-mixed with the respective local signals of the frequency synthesizers 17 and 18 in the mixers 15 and 16. Then, the frequency-converted output signals are filtered by the bandpass filters 19 and 21. The output signals filtered from the bandpass filters 19 and 21 are amplified by the automatic gain control applifiers 22 and 23, and thereafter quadrature-detected by the quadrature detecting circuit 24 to obtain the baseband signals of the I channel and the Q channel. The respective I/Q-channel baseband signals derived from the quadrature detecting circuit 24 are A/D-converted by the A/D converters 26 and 27 into the digital baseband signals which are supplied to the demodulating circuit 28 for demodulation purposes. Similarly, the respective I/Q-channel baseband signals derived from the quadrature detecting circuit 25 are A/D-converted into the digital baseband signals by the A/D-converters 29 and 31 which are supplied to the demodulating circuit 32 for demodulation purposes. The output signals filtered from the bandpass filters 19 and 21 are separated and supplied to the level detecting circuits 33 and 34, so that the reception levels thereof are detected. In the TDMA transmission system during the empty time while the signal of the receiving channel is not transmitted, the signals of other channels can be received and the levels of these received signals can be measured. To the contrary, the receiver using the conventional FDMA transmission method cannot measure the levels of the other channels with the different frequencies. To monitor the received signal levels of the peripheral zones during the communication operation so as to switch the channels, two series of the signal receiving units are required. As a result, the scale of the receiving circuit employed in the mobile station in the conventional FDMA transmission system is increased approximately twice as that of the TDMA transmission system. As to the scale, the conventional FDMA transmission system is inferior to the TDMA transmission system. In the conventional radio communication apparatus, the frequency of the carrier signal is switched by the frequency synthesizer operable in the RF frequency band. As a consequence, no information contained in the carrier signals with the different frequencies can be transmitted and also received in the duplication mode. If the signal transmission and reception are carried out with employment of a plurality of different carrier signals in the conventional FDMA type radio apparatus, then two sets of the transmission unit and the reception unit should be necessarily required. US-A-5 245 340 discloses of prior art receiver for receiving FDMA signals. As is well known in the mobile communication systems, such severe requirements of low power consumption and compactness are required for the mobile station. In view of these severe requirements, two sets of the transmission/reception units would cause serious demerits. Therefore, the present invention has been made in an attempt to alleviate the above-described demerits, and in one aspect provides a communication apparatus and a communication system, capable of detecting a level of a carrier signal present in other signal channel than the present signal channel under communication, without employing two series of signal reception units. According to another aspect, the present invention provides a communication apparatus and a communication method, capable of receiving a signal from a plurality of signal channels without using two series of signal reception units. According to a first aspect of the present invention, there is provided a receiver for receiving an FDMA signal comprising a modulation signal including two carrier frequencies into an intermediate frequency signal; a quadrature detecting circuit for converting said intermediate frequency signal into a baseband signal of an I channel and a baseband signal of a Q channel; and A/D converting means for A/D converting said baseband signals of the I/Q channels into digital signals; the receiver being characterised by: a frequency converting element structured of two frequency converting circuits arranged to frequency convert said digital signals into two respective output signals whose centre frequencies are zero, corresponding to said two carrier frequencies, respectively. According to a second aspect of the present invention, there is provided a method of receiving an FDMA comprising a modulation signal including two carrier frequencies into an intermediate frequency signal; a quadrature detecting circuit for converting said intermediate frequency signal into a baseband signal of an I channel and a baseband signal of a Q channel; and A/D converting means for A/D converting said baseband signals of the I/Q channels into digital signals; the receiver being characterised by: a frequency converting element comprising a memory for storing said digital signals and a circuit arranged to generate from said digital signals read out from said memory, two output signals whose centre frequencies are zero, corresponding to said two carrier frequencies, in a time division mode. According to a third aspect of the present invention there is provided a method of receiving an FDMA signal comprising the steps of: frequency converting the received input FDMA signal comprising a modulation signal including two carrier frequencies into an intermediate frequency signal; converting said intermediate frequency signal into a baseband signal of an I channel and a baseband signal of a Q channel; and A/D converting said baseband signals of the I/Q channels into digital signals; the method being characterized by converting said digital signals using two frequency converting circuits to generate two respective output signals whose centre frequencies are zero, corresponding to said two carrier frequencies. According to a fourth aspect of the present invention there is provided A method of receiving an FDMA signal comprising the steps of: frequency converting the received input FDMA signal comprising a modulation signal including two carrier frequencies into an intermediate frequency signal; converting said intermediate frequency signal into a baseband signal of an I channel and a baseband signal of a Q channel; and A/D converting said baseband signals of the I/Q channels into digital signals; the method being characterised by: storing said digital signals in a memory; and generating from said digital signals read out from said memory, two output signals whose centre frequencies are zero; corresponding to said two carrier frequencies, in a time division mode. The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments of the invention taken in conjunction with the accompanying drawings : Fig. 1 is a schematic explanatory diagram for showing a structure of a transmission frame employed in the FDMA transmission system; Fig. 2 schematically shows the circuit block diagram of the conventional receiving circuit of the mobile station used in the FDMA transmission system; Fig. 3 schematically illustrates a circuit block diagram of a receiving circuit employed in a communication apparatus according to a first preferred embodiment of the present invention; Fig. 4A is a schematic block diagram for representing internal circuit arrangements of the frequency converting circuits 43 and 44 employed in the receiving circuit of Fig. 3; Fig. 4B explanatorily shows the frequency converting method employed in the receiving circuit of Fig. 3; Fig. 5 schematically shows a circuit block diagram of another receiving circuit employed in a communication apparatus according to a second preferred embodiment of the present invention; Fig. 6 schematically indicates a circuit block diagram of another receiving circuit employed in a communication apparatus ; and Fig. 7 is an illustration for explaining a hand over effected among carrier signals having different frequencies in the communication network system with employment of the communication apparatus according to the present invention. Referring to the accompanying drawings, FDMA communication method/apparatus/system according to the present invention will be described in detail. [FIRST EMBODIMENT] In Fig. 3, there is shown an overall circuit arrangement of a receiving circuit employed in an FDMA communication system, according to a first preferred embodiment of the present invention. It should be noted that the same reference numerals shown in Fig. 2 are employed as those for denoting the same or similar elements represented in the following drawings. In the first receiving circuit of Fig. 3, an input signal received at the input terminal 11 is not subdivided into two sets of input signal, but is directly supplied to a bandpass filter (BPF) 13. Then, the input signal filtered by this bandpass filter 13 is frequency-mixed in a mixer 15 with a local oscillation signal having a fixed frequency, derived from a fixed (local) oscillator 41. As a result, the input RF (radio frequency) signal is converted into an intermediate frequency signal, while being multiplexed on the frequency domain. In other words, these mixer 15 and bandpass filter 19 constitute a down-converting circuit. The intermediate frequency (IF) signal is amplified in a linear fashion by a predetermined signal level in an automatic gain control (AGC) circuit 22. Furthermore, the gain-controlled IF signal is quadrature-detected by a quadrature detecting circuit 24 in such a manner that, for instance, this IF signal is quadrature-detected to produce I and Q channels, while one-end frequency over the entire band is used as a local signal. These I/Q channel signals are mutually offset by such frequency components corresponding to the respective carrier signals contained in a plurality of input RF signals. The detection signals of the I/Q channels are A/D-converted by A/D converters 26 and 27 into digital signals, respectively. Then, the digital signals are frequency-converted by corresponding frequency converting circuits 43 and 44, so that such signals contained in the input RF signal, corresponding to two channels of interest, are outputted as signals whose center frequencies are zero. Thereafter, these frequency-converted signals are supplied to a demodulating circuit 28 and a level detecting circuit 45, respectively. In one frequency converting circuit 43, the digital quadrature-detected signal in the channel under communication operation is converted into I/Q baseband signals whose center frequencies are zero. When a level detection of a designated carrier signal of the peripheral zone is carried out, this designated carrier signal is converted into I/Q baseband signals whose center frequencies are zero in the other frequency converting circuit 44. Then, these I/Q baseband signals are processed in the level detecting circuit 45 in such a manner that a calculation is made of square summation (I 2 + Q 2 ) of the respective amplitudes of these I/Q baseband signals. An internal circuit arrangement of the frequency converting circuit 43 is constructed as, for instance, shown in Fig. 4A. As seen from Fig. 4A, the digital signals cos[Φ n ±2πΔ fit ] and sin[ Φ n ±2πΔ fit ] derived from the A/D converters 26 and 27 are multiplied with digital local signals cos ( ±2πΔ fit ) and sin( ±2πΔ fit ) derived from local signal generators 46 by multipliers 47 and 48, respectively. The multiplied output signals are supplied via lowpass filters 51 and 52 to a frequency correcting circuit 53. As indicated in Fig. 3, the local oscillator 41 is a fixed oscillator. Also, the local signal generators 45 and 46 may change the local signal frequencies, but does not follow the carrier frequency of the channel of interest for this input intermediate frequency signal, so that the quasi-synchronization type quadrature detection is carried out. As a result, the quadrature-detected signals contain frequency errors and fixed phase errors. These errors may be corrected by way of the digital processing in the frequency correcting circuit 53. It should be noted that the frequency conversion by way of this digital processing may be realized by employing the frequency converting filter as described in Japanese Patent Application No. 4-199018 (filed in 1992) corresponding to published application JP-A-6-21722. For instance, as illustrated in Fig. 4B, the frequency converting manner is explanatorily performed. That is, the low frequency signal is previously removed by the lowpass filter with respect to the signal components of the quadrature-detected signal, which have been frequency-offset (see Fig. 4B-a). Then, the lowpass-filtered signal components are sampled at this center frequency in correspondence with each of these offset frequencies, so that the folded signal components whose center frequencies are zero are produced (see Fig. 4B-b). Finally, the folded signal components are derived from the lowpass filter (see Fig. 4B-c). This arrangement may be constituted by the same filter by varying the sampling frequency in accordance with the frequencies of the respective channels. [SECOND EMBODIMENT] Fig. 5 schematically shows a circuit arrangement of another receiving circuit employed in the communication apparatus, according to a second preferred embodiment of the present invention. A major feature of this receiving circuit according to the second embodiment is such that the digital signals derived from the A/D converters 26 and 27 are temporarily stored into a memory circuit 55, and these digital signals are read out from this memory circuit 55 at the reading speed two times higher than the writing speed, and thereafter the read digital signals are supplied to a frequency converting circuit 56. In this frequency converting circuit 56, the same frequency converting process as in the above-described frequency converting circuit 43 shown in Fig. 3 is performed in synchronism with the even-numbered reading clocks of the memory circuit 55, whereas the same frequency converting process as in the above-mentioned frequency converting circuit 44 indicated in Fig. 3 is carried out in synchronism with the odd-numbered reading clocks of this memory circuit 55. The output signal from the frequency converting circuit 56 is supplied to the demodulating circuit 28 and the level detecting circuit 45. In this demodulating circuit 28, the demodulating process is performed in synchronism with the even-numbered reading clocks of the memory circuit 55. On the other hand, the level detecting process by the level detecting circuit 45 is performed in synchronism with the odd-numbered reading clocks of the memory circuit 55. [THIRD EMBODIMENT] Fig.6 is a schematic circuit block diagram of another receiving circuit for mobile station having plurality of carrier signals, for performing the CDMA (code division multiple access) transmission, according to a third preferred embodiment of the present invention. In this receiving circuit, the spread signals having the different carrier frequencies are first converted into baseband signals whose center signals are zero by the frequency converting circuits 43 and 44. These baseband signals are inverse-converted into narrow band signals by correlation detecting units 57 and 58 based on the spread codes corresponding to the respective received signals. It should be noted that although two signal series of the basebend signal processing circuits have been employed in the receiving circuit of Fig.6, alternatively only one signal series of the baseband signal processing circuit may be employed to perform the baseband signal processing operations for several channels succeeding to the frequency converting circuits under such conditions that, as previously explained with respect to the second embodiment these spread signals are once stored into the memory circuit, and then are processed in the time division method by utilizing the highspeed clock. [FOURTH EMBODIMENT] In. Fig. 7, there is shown a system arrangement of a network system with employment of the communication apparatus according to the present invention. In this network system, when a mobile station 140 is moved from a zone of one base station 120 to a zone of the other base station 130, hand over (channel switching) is performed. Conventionally, since the carrier signals are switched during this hand over, the communication is instantaneously interrupted. To perform such hand over without instantaneously interrupting the communication, it is conceivable that the same information is transmitted from a plurality of base stations 120 and 130 to the same mobile station 140 by utilizing the carrier signals f1 and f2, respectively. In the up link channel, the base stations 120 and 130 receive the electromagnetic wave transmitted from the same mobile station 140, plural pieces of information are synthesized in a control station 110. In accordance with the communication apparatuses and the communication methods described in connection with the first to third embodiments, the frequency conversion with respect to the carrier signals having the plural frequencies can be performed without employing two signal processing paths of the receiving circuit. As previously described in connection with the first to third embodiments, one of the plural carrier signals is demodulated and the level of an other carrier signal is detected. To the contrary, in accordance with this fourth embodiment, a demodulating circuit (not shown) is employed to substitute for the level detecting circuit 45 shown in Fig. 3 and Fig. 5. As a consequence, both the control station 110 and the mobile station 140 may demodulate a plurality of carrier signals without employing two signal processing paths of the receiving circuit. Also, as previously explained in the third embodiment even when the baseband signal processing for several channels is performed by the single series of baseband signal processing circuit in the receiving circuit of Fig. 6, a demodulating circuit (not shown either) is employed instead of the level detecting circuit 45 shown in Fig. 6, so that the demodulation for the plural carrier signals can be performed in a similar manner. As the method for synthesizing a plurality of signals transmitted from the plural base stations, it is possible to utilize such a maximum ratio synthesis that these transmitted signals are weighted in response to the levels of these signals and then the weighted signals are summed with each other. As a consequence, the highest diversity effect can be achieved, thereby improving the communication quality. In the up link channel, the mobile station 140 transmits the same information to the base stations 120 and 130 by using a plurality of carrier signals. In the transmitting circuit, a modulating circuit (not shown) is employed to substitute for the demodulating circuit employed in the receiving circuit, and a D/C converter is employed to substitute for the A/D converters 26 and 27. A plurality of digital signals outputted from the frequency converting circuits 43 and 44 are added to each other, and then the added digital signals are inputted into this D/A converter. Similar to the operations described in the embodiment 2, the plural carrier signals are frequency-converted in the time division manner by employing a single frequency converting circuit 56, and then two digital signals which have been frequency-converted may be added to each other. To synchronize two carrier signals outputted in the time division manner, the memory circuit 55 is employed similar to the second embodiment, and then the signals stored in this memory circuit 55 may be added to each other. The different carrier signals transmitted from the mobile station 140 are received by the plural base stations 120 and 130, and are diversity-synthesized with each other in the control station 110. As a result, the communication quality in the up link channel can be improved. As previously described, only a few number of circuit is additionally employed in the conventional FDMA transmission radio apparatus, whereby the levels of the channels of the carrier frequencies other than the channel under communication can be detected without employing two signal receiving circuit paths. Moreover, according to the communication apparatus of an embodiment of the present invention, the signals can be received with respect to the plural channels without employing two signal processing paths of the receiving circuit.

Claims: A receiver for receiving an FDMA signal comprising: a down converting circuit (15, 19) for frequency converting the received input FDMA signal, comprising a modulation signal including two carrier frequencies into an intermediate frequency signal; a quadrature detecting circuit (24) for converting said intermediate frequency signal into a baseband signal of an I channel and a baseband signal of a Q channel; and A/D converting means (26, 27) for A/D converting said baseband signals of the I/Q channels into digital signals; the receiver being characterised by : a frequency converting element (43, 44; 43, 44, 57, 58) structured of two frequency converting circuits (43, 44) arranged to frequency convert said digital signals into two respective output signals whose centre frequencies are zero, corresponding to said two carrier frequencies, respectively. A receiver for receiving an FDMA signal comprising: a down converting circuit (15, 19) for frequency converting the received input FDMA signal, comprising a modulation signal including two carrier frequencies into an intermediate frequency signal; a quadrature detecting circuit (24) for converting said intermediate frequency signal into a baseband signal of an I channel and a baseband signal of a Q channel; and A/D converting means (26, 27) for A/D converting said baseband signals of the I/Q channels into digital signals; the receiver being characterised by : a frequency converting element (55, 56) comprising a memory (55) for storing said digital signals and a circuit (56) arranged to generate from said digital signals read out from said memory (55), two output signals whose centre frequencies are zero, corresponding to said two carrier frequencies, in a time division mode. A receiver as claimed in claim 1 or claim 2, further comprising: a demodulating circuit (28) for demodulating one of said two output signals; and level detection means (45) for detecting the level of the other signal of said two output signals. A receiver as claimed in claim 1, further comprising: a first demodulating circuit (28) for demodulating one of said two output signals; and a second demodulating circuit for demodulating the other signal of said two output signals. A method of receiving an FDMA signal comprising the steps of: frequency converting the received input FDMA signal comprising a modulation signal including two carrier frequencies into an intermediate frequency signal; converting said intermediate frequency signal into a baseband signal of an I channel and a baseband signal of a Q channel; and A/D converting said baseband signals of the I/Q channels into digital signals; the method being characterised by : converting said digital signals using two frequency converting circuits (43, 44) to generate two respective output signals whose centre frequencies are zero, corresponding to said two carrier frequencies. A method of receiving an FDMA signal comprising the steps of: frequency converting the received input FDMA signal comprising a modulation signal including two carrier frequencies into an intermediate frequency signal; converting said intermediate frequency signal into a baseband signal of- an I channel and a baseband signal of a Q channel; and A/D converting said baseband signals of the I/Q channels into digital signals; the method being characterised by : storing said digital signals in a memory (55); and generating from said digital signals read out from said memory (55), two output signals whose centre frequencies are zero, corresponding to said two carrier frequencies, in a time division mode. A method as claimed in claim 5 or 6, further comprising the steps of: demodulating one of said two output signals; and detecting level of the other signal of said two output signals. A method as claimed in claim 5, further comprising the steps of: demodulating one of said two output signals; and demodulating the other signal of said two output signals.

Inventors: ADACHI FUMIYUKI, DOHI TOMOHIRO, SAWAHASHI MAMORU, ADACHI, FUMIYUKI, DOHI, TOMOHIRO, SAWAHASHI, MAMORU
Applicants: NTT DOCOMO INC, NTT DOCOMO, INC.
Application year: 1994
Main classification: H04B17
IPC codes: H04B1, H04B17